1. Field of Invention
This invention relates generally to a technique for enhancing the electrical characteristics of thick film resistors incorporated in microelectronic chips, and more particularly to a technique for this purpose which acts to effect an increase and then a decrease in the ohmic value of a thick film resistor to bring it to its target value, in the course of which treatment the characteristics of the resistor are significantly enhanced.
2. Status of Prior Art
Microelectronics is that branch of the electronics art which deals with extremely small components, assemblies or systems. In one well-known form of microelectronic structure, resistors, capacitors and conductors are formed by depositing chemical materials onto the surface of a substrate to define a "thin-film" circuit. In another form, a substrate is also employed, but resistors and conductors are printed onto its surface, all other circuit components, such as capacitors, diodes, etc., being discrete elements. This type of microelectronic structure is known as "thick-film" or a ceramic printed circuit. Ceramic printed circuits are the main concern of the present invention, for these may be inexpensively mass produced, and, because of their compactness, light weight and low cost, they are widely used in many forms of modern electronic equipment.
Thick film resistors are made by deposition on an insulating substrate such as alumina, using screening, firing or pyrolytic deposition techniques. Screening is the most commonly used technique, a resistive paste being squeezed onto the substrate through a stainless steel or silk mesh. To stabilize the ohmic value of the resistor, after it is dried, the resistor is fired, typically for about 30 minutes at 600.degree. to 900.degree. C. Commonly used thick films are usually mixtures of palladium or ruthenium with conductive metals such as silver or gold. The absolute value tolerances of a typical thick film resistor lie in a range of .+-.20.degree. (Roydn D. Jones, Hybrid Circuit Design and Manufacture, Marcel Dekker, Inc., Publishers, 1982).
While this fabrication technique gives rise to resistance values which are fairly close to the required tolerances, it is still necessary to make a final adjustment, for it is not possible to lay down precision resistors. When close tolerances are required, resistors are often designed for about 80% of their target value and then trimmed to the desired value. With existing abrasion trimming methods using air or sandblasting, one percent tolerance is achievable by the physical removal of resistive material embedded in the resistor deposit following the firing cycle. Removal of this material from the edge of the printed resistor by an abrasion unit gives positive control of precision resistance values.
Nevertheless, the abrasion technique for trimming resistors has many serious drawbacks, for it not only degrades or destroys the physical qualities of the resistors, but it also reduces their physical dimensions, with an accompanying loss in power-handling capacity. Moreover, the abrasion technique is capable only of effecting an increase in resistance value so that if the resistor value, as printed, is initially too high, it is not correctable and the resistor must be rejected.
Another known thick film trimming technique is oxidation in which the resistor films are treated in an oxidizing atmosphere, causing some of the material to become oxidized, thereby increasing the total resistance value.
In the laser trimming technique which is now widely used, a small portion of the film resistor is selectively evaporated to increase its effective resistance. In this procedure, a focused laser beam is first moved at right angles to the resistor for coarse trimming, and as the resistor approaches its final value, the beam is moved parallel to the resistor length for fine adjustment.
In projecting a jet of sand or other abrasive material against the resistor surface, it is difficult to control the degree of attrition, as a consequence of which the ohmic value may be caused to rise beyond the desired tolerance. Since correction can only be effected unidirectionally, in the event the trimming action overshoots the desired value, the resistor is no longer correctable and must be rejected. Thus, printed resistors which initially are too high in value or which have been excessively trimmed are beyond correction with existing abrasion trimming techniques. This drawback is also experienced in laser trimming techniques which act to raise the ohmic value of a thick film resistor but cannot reduce the value.
A single defective resistor in a ceramic printed circuit renders the entire circuit unacceptable, and a mistake in trimming one resistor in a printed circuit assembly makes it necessary to reject the entire circuit. The likelihood of a single error is particularly great when the assembly includes a large number of resistors such as in a ladder network. In practice, therefore, with existing trimming techniques, the rejection rate is quite high. This factor raises manufacturing costs substantially.
U.S. Pat. Nos. 3,676,633 and 3,646,684 (hereinafter referred to as the Di Mino patents) disclose an electronic technique for trimming the ohmic value of a resistor included in a microelectronic circuit to effect a correction in either direction with respect to the initial value of the resistor without, however, changing its physical dimensions.
Apparatus for this purpose disclosed in these Di Mino patents includes a high-frequency oscillator having a resonator coil to produce an R-F carrier, the oscillator being modulated by an audio-frequency signal to create pulsatory R-F energy. The resonator coil is inductively coupled to a step-up coil connected to a "Down" probe which produces a corona discharge beam that when directed to a point on the resistor acts to reduce its ohmic value. The resonator coil is also inductively coupled to a step-down coil connected to an "Up" probe which when brought into physical contact with a point on the resistor produces a current flow therein that acts to increase the value of the resistor. The extent of such ohmic change is determined by the duration of the treatment and by the area of the resistor subjected to treatment.
At the time the Di Mino patents were granted, the dimensions of a typical resistor network chip or wafer containing resistors that required trimming was approximately one square inch, and the only way one could trim these resistors was to bring the chip into the close proximity of the probes to perform the required trimming operations. Currently, however, the typical resistor network chip is greatly reduced in size and is in many cases about a tenth of the size of the chips previously produced. One cannot as a practical matter handle these tiny chips to effect trimming by the apparatus disclosed in the Di Mino patents.
Modern production procedures make use of servo mechanisms or X-Y positioning tables to position a device to be worked on relative to a tool or other apparatus to carry out the work. Apparatus of the type disclosed in the Di Mino patents does not lend itself to incorporation in a servo mechanism or an X-Y positioning table, for the apparatus is relatively massive and cannot readily be manipulated.
The above-identified copending Di Mino patent application discloses a system for trimming the ohmic value of a resistor included in a microelectronic chip to raise or lower its value without changing its physical dimensions, which system makes it possible to manipulate the Up and Down probes so that they can be brought into operative relation to the chip, thereby dispending with the need to handle the chip to bring it into the proximity of the probes. A significant advantage of this system is that it lends itself to use with servo mechanisms or X-Y positioning tables on a production line to carry out resistor trimming operations at a relatively rapid rate.
The system disclosed in the Di Mino copending application is provided with a self-contained unit which generates a low radio-frequency carrier overmodulated by a sonic frequency signal to yield at its output terminal pulsed radio-frequency energy. Coupled by an extension cable to the output terminal of the unit is a portable probe assembly whose position may be manipulated to bring the assembly into operative relation with the resistor to be trimmed.
The portable assembly includes a tank circuit connected by the cable to the output terminal of the unit and tuned to the carrier frequency so that the pulsed energy is stored therein and not radiated. A "Down" probe is inductively coupled in step-up relation to the tank circuit to cause a corona discharge beam to be projected from the tip of this probe, which beam when directed toward a point on the resistor acts to irradiate the resistor to reduce its ohmic value. An "Up" probe is inductively coupled in step-down relation to the tank circuit so that when this probe is brought into contact with a point on the resistor, the resultant heating current acts to increase its ohmic value.
The concern of the present invention is with the parameters of thick film resistors; that is, their significant electrical characteristics. These electrical characteristics are the resistance stability of the resistor (ohms per square), its temperature coefficient of resistance (ppm/.degree. C.), its voltage coefficient of resistance, and its noise level. These characteristics leave much to be desired in manufacturing thick film resistors and they are not improved by known trimming techniques and indeed are degraded in the case of abrasion trimming.